Non-aqueous electrolyte secondary batteries, particularly lithium secondary batteries, have high electromotive force and high energy density, so they are widely used as the main power source for mobile communication devices and portable electronic devices. Also, there is an increasing demand for non-aqueous electrolyte secondary batteries as memory back-up power sources. Portable electronic devices have been undergoing remarkable developments and are becoming increasingly smaller in size, increasingly higher in performance, and free of maintenance, thereby creating a strong demand for non-aqueous electrolyte secondary batteries with higher energy density.
From the viewpoint of heightening the capacity of lithium secondary batteries, silicon materials have been receiving attention as negative electrode materials with a larger theoretical capacity than that of carbon materials. The theoretical capacity of silicon is larger than those of graphite, aluminum and the like.
However, crystalline silicon undergoes volume changes when it absorbs or desorbs lithium ions during charge/discharge, and the rate of such volume expansion is about four times at maximum. Thus, when silicon is used as an active material, it becomes pulverized due to distortion upon volume change, so that the electrode structure is destroyed.
Japanese Laid-Open Patent Publication No. 2000-243449 proposes the use of a silicon oxide as a negative electrode active material. When a silicon oxide absorbs lithium ions, its structure changes into a microcrystalline structure composed of a lithium-silicon alloy and a lithium oxide. As a result, even when it undergoes volume changes and distortion, its strength is increased and its pulverization is suppressed. On the other hand, when the negative electrode active material releases lithium ions until it is fully discharged, the lithium-silicon alloy changes into silicon and the lithium oxide does not change. However, in actual use of batteries, the negative electrode active material is not fully discharged. Therefore, when the negative electrode active material is in a discharged state, it is composed of a lithium-silicon alloy with a reduced lithium content and a lithium oxide.
Clock back-up power sources for digital still cameras, etc. are often left in the device, but do not usually have a protective circuit or the like to cut discharge. As a result, such back-up power sources are often discharged (deeply discharged) until the battery voltage lowers to near 0 V. Hence, cycle life characteristics in charge/discharge cycles including deep discharge (hereinafter referred to as deep discharge cycles) are particularly important for back-up power sources.
When a manganese oxide is used as a positive electrode active material and graphite is used as a negative electrode active material, if such a battery is discharged at a low rate such as about 0.1 C (1 C is a 1 hour-rate current) until the battery voltage lowers to near 0 V, the positive electrode and the negative electrode have almost the same potential due to small polarization. In such cases, the potential range in which the positive electrode has good reversibility (1.8 V or higher relative to lithium metal (Li/Li+)) is not compatible with the potential range in which the negative electrode has good reversibility (0.2 V or lower relative to lithium metal (Li/Li+)). When the battery is deeply discharged, at least one of the positive electrode active material and the negative electrode active material deteriorates, thereby resulting in significant degradation of cycle life characteristics.
Japanese Laid-Open Patent Publication No. Hei 7-335201 proposes mixing a lithium-containing compound that releases lithium ions at potentials not more than 2 V with a carbon material (negative electrode active material), in order to prevent dissolution of a negative electrode current collector during deep-discharge. This proposal is effective for batteries including a copper negative electrode current collector. However, batteries for use as clock back-up power sources usually employ a molded negative electrode mixture (pellet) as the negative electrode and thus do not have a negative electrode current collector.
From the viewpoint of preventing deep-discharge of a specific battery in a set of batteries that are connected in series, Japanese Laid-Open Patent Publication No. 2001-243943 proposes reducing the voltage change rate of each battery to suppress variations in end-of-discharge voltage. However, this proposal relates to a plurality of batteries that are connected in series and requires a specific system for controlling discharge voltage.
When a silicon oxide is used as a negative electrode active material as proposed in Japanese Laid-Open Patent Publication No. 2000-243449, the deep discharge cycle life becomes short. It has been found that repetitive deep discharge cycles cause a sharp increase in the amount of coating film formed on the active material surface.
The reason for the sharp increase in the amount of coating film is probably that deep discharge makes the distribution of electronic resistance in the negative electrode active material uneven, thereby causing the reaction to proceed unevenly. If the reaction proceeds intensively in a certain region, a large amount of coating film is produced and the deterioration of the region proceeds. As a result, the reaction proceeds intensively in another region, so that the coating film formation and the deterioration proceed acceleratedly.
The reason for the creation of uneven distribution of the electronic resistance in the negative electrode active material is that when deeply discharged, the silicon oxide has a significantly increased electronic resistance. In the silicon oxide in a deeply discharged state, the lithium-silicon alloy produced during charge has a reduced lithium content, so that the alloy has an increased resistance.
When a manganese oxide and a silicon oxide are used as a positive electrode active material and a negative electrode active material, respectively, in a battery that is designed such that the negative electrode capacity is larger than the positive electrode capacity, the potential of the positive electrode relative to lithium metal (Li/Li+) in a deeply discharged state is lower than 1.8 V. As a result, the crystal structure of the manganese oxide may undergo an irreversible change.